In the rating system of the Royal Navy used to categorise sailing warships, a sixth-rate was the designation for small warships mounting between 20 and 28 carriage-mounted guns on a single deck, sometimes with smaller guns on the upper works and sometimes without. It thus encompassed ships with up to 30 guns in all. In the first half of the 18th century the main battery guns were 6-pounders, but by mid-century these were supplanted by 9-pounders. 28-gun sixth-rates were classed as frigates, those smaller as 'post ships', indicating that they were still commanded by a full ('post') captain, as opposed to sloops of 18 guns and less, which were under commanders.

The rating system of the Royal Navy and its predecessors was used by the Royal Navy between the beginning of the 17th century and the middle of the 19th century to categorise sailing warships, initially classing them according to their assigned complement of men, and later according to the number of their carriage-mounted guns. The rating system of the Royal Navy formally came to an end in the late 19th century by declaration of the Admiralty. The main cause behind this declaration focused on new types of gun, the introduction of steam propulsion and the use of iron and steel armour which made rating ships by the number of guns obsolete.